lux to lumen BH1750 I2C light sensor

Question

my BH1750 I2C light sensor is giving me a reading in lux but I need a lumen-value. From what I read I just multiply the lux-reading by the surface area of the sensor to get my lumen-value. But from the datasheet on page 6 I'm getting a very small surface area of 0.25mm by 0.3mm. That doesn't seem right. What am I doing wrong? I'm getting a reading of about 8,000 lx on this cloudy afternoon which should be somewhere around 600 lumens.

Answer 1

You seem to have a wrong understanding of photometric quantities. Let me try to get this straight with an analogon: Consider a water fountain that emits water. This fountain will stand for our light source.

The total amount of water that the fountain emits can be measured as m³/s (cubic meters per second). This is a characteristic of the fountain, which could be called the water power. Going back to photometry, this power is equivalent to the luminous flux, which is measured in lumen. Therefore, the luminous flux describes how much light a light source emits. You can restrict this definition to a given set of directions (e.g., measure the luminous flux of a light bulb only in a downward cone). This will give you the total amount of light that travels in that cone. For the fountain example, this can be done equivalently. Just measure the water emitted into a given cone. The unit is still m³/s or lumen.

Now, let us not only consider the fountain (as the light source) but also the surrounding surfaces. We can pick an arbitrary point on the surrounding surface (a point with no area) and measure how much water/light arrives at this point. This might be a bit hard to imagine because it is a differential quantity. You can approximate this value by measuring the amount of water that arrives in a small neighborhood of the point and dividing by the area of that neighborhood. This is actually what your sensor is doing. The resulting unit is m³/s/m² (cubic meters per second per square meter) or for the photometric case lm / m² (lumen per square meter), which is the definition of lux (unit of illuminance). Therefore, different points can have different illuminance. Especially, points far away from the light source usually have a smaller illuminance. You can calculate the total luminous flux by integrating the illuminance of the entire surface area. This is equivalent to measuring the amount of water at very many small surface pieces around the fountain (i.e. illuminance multiplied by area) and summing them up.

With this background knowledge, we see that it does not make sense to convert lux to lumen. They measure completely different things. Intuitively, illuminance tells you how much light shines at a given point, which is usually what you want. What you did (by multiplying the illuminance by the sensor area) is calculating the total luminous flux that arrives at the sensor (total amount of water at a given surface patch). Naturally, this measure grows as your sensor gets bigger (there will be more light arriving at the surface; or equivalently, as you consider bigger and bigger patches around the fountain, you will collect more and more water). Therefore, it does also not make sense to state that 8 klx should be 600 lm.